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xen-devel
Re: [Xen-devel] A question no one can answer
Hi Robert,
The memory bus controller like Memory Controller Hub (MCH) on Intel's north bridge should arbitrate the traffic from different cores/processors. I guess the hardware can guarantee some kind of fairness so that no core would be starving and the other one is busy. (just my guess, I don't know the details)
However, control the bandwidth with priorities is difficult. Any one has ideas to this problem?
Weiming
On Thu, Feb 14, 2008 at 10:24 PM, Robert Stober < rstober@xxxxxxxxxxxx> wrote:
Weiming,
I agree that it is very hard, and that no one has done it.
But nevertheless I suggest the following question to the Xen
developers:
Given the fact that memory bandwidth is shared amongst
multiple cores on a single die, assume that one VM is running on each core. What
is to stop one VM from saturating the memory bus, causing reduced performance of
all the other VMs? This is the general multi-core problem, not specific to
Xen. But it affects Xen greatly. What use is it to allocate memory to a VM if it
can't use the memory because a process of another VM has saturated the memory
bus?
Thank you,
Robert
You mean QoS on memory bandwidth? It's an interesting question.
(there are similar problems like QoS on shared L2) I think it's hard to
do. Even for a native OS, it may only control this indirectly by
adjusting the CPU time slice. And I don't think any commercial OS has
implemented such feature yet. Weiming
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